The invention relates to a vehicle master cylinder connected to an auxiliary braking device. What an auxiliary braking device means is a device grafted on to the master cylinder with the purpose of tapping hydraulic fluid from this same master cylinder. In one example, the auxiliary braking device may be of the E.S.P. (Electronic Stability Program known in French as “Correcteur Electronique de Trajectoire”) type or the A.B.S. (Antilock Braking System, known in French as “système d'antiblocage de freinage”) type. The object of the invention is to obtain a master cylinder with a low production cost. The invention is more specifically intended for the field of motor vehicles but could be applied to other fields.
The master cylinder allows a hydraulic fluid or brake fluid to be sent under pressure toward the brakes of a vehicle, the objective of this being to retard the vehicle, or even to bring the vehicle to a standstill by blocking the rotation of at least one of the wheels of the vehicle. Tandem master cylinders comprising a primary piston and a secondary piston, each one sliding in one bore of the master cylinder, are known. The primary piston and the secondary piston respectively define a primary chamber and a secondary chamber within a bore. The primary chamber and the secondary chamber are respectively connected to a primary hydraulic reservoir and to a secondary hydraulic reservoir. Each of the reservoirs contains brake fluid to fill the corresponding chamber.
The primary reservoir and the secondary reservoir communicate with the primary chamber and the secondary chamber respectively via a primary duct and a secondary duct. The primary duct and the secondary duct are created in a body of the master cylinder. The primary duct and the secondary duct open into the bore of the master cylinder.
The primary piston and the secondary piston comprise a primary port and a secondary port respectively. Each of these ports is formed in the material of which the corresponding piston is formed. Each of these ports opens via a first end facing a wall delimited by the bore of the master cylinder and via a second end into a corresponding chamber.
Positioned along the wall of the master cylinder are a primary seal and a secondary seal. The primary seal and the secondary seal control the flow of brake fluid from the primary reservoir and the secondary reservoir to the primary chamber and the secondary chamber respectively via the primary port and via the secondary port. The primary seal and the secondary seal control the flow of brake fluid according to the position of the primary piston and according to the position of the secondary piston within the bore.
As already mentioned, auxiliary braking devices which are grafted on to the master cylinder are known. These auxiliary braking devices tap brake fluid from the master cylinder. The E.S.P. controls the rotation of each of the wheels of the vehicle independently of one another. The A.B.S. prevents one wheel of the vehicle from locking up when confronted with a particular obstacle.
To operate the E.S.P. device for example, the brake fluid may be tapped directly from one of the brake fluid reservoirs of the master cylinder. In order to tap off this brake fluid, an opening may be made through this reservoir. Now, this opening may cause brake fluid leaks or pressure drop. Such leaks or such pressure drops may lead to slower braking. Alternatively, such leaks or such pressure drops may lead to difficulty in bringing the vehicle quickly to a standstill.
It is also possible to tap this fluid from the primary duct and/or from the secondary duct. To do this, a passage cross section of each of these ducts may be increased in order to obtain a sufficiently high flow rate. However, by increasing a passage cross section such as this, there is the risk that the dead travel of the corresponding piston will be lengthened. What the dead travel of the corresponding piston means is the time taken by the piston to move from an initial rest position into a position in which the brake fluid contained in the corresponding chamber begins to rise in pressure.
Machining channels along the wall of the master cylinder is also known. These channels are created in the form of helixes along this wall. Such channels may be filled with brake fluid which can then be tapped off by the E.S.P. device. However, creating such channels along the wall of the master cylinder presents problems with manufacturing, particularly since creating such helixes appreciably increases the cost of manufacture of such a master cylinder.
In order to solve these problems, the invention plans to hollow out the piston radially over a circumference of the piston so as to form an indentation. This indentation is created upstream of the port in a direction of forward travel of the piston during the braking phase.
As a preference, the piston comprises a first indentation and a second indentation that follow on from one another in the direction of forward travel of the piston during the braking phase. These first and second indentations are created in such a way that a reduction in a diameter of the piston is obtained in the direction of forward travel of the piston in the braking phase. The seal is forced to collaborate with the piston by being positioned bearing against the first indentation and against the second indentation according to the various positions of the piston within the bore.
This first indentation and this second indentation free up space between the corresponding piston and the wall of the master cylinder. Brake fluid can be housed in this space so that it can be tapped off by the E.S.P. device as already described hereinabove.
The first indentation allows the E.S.P. device to be supplied with fluid right from the start of a return of the corresponding piston to the rest position. The second indentation allows the brake fluid flow rate to be high enough to supply the E.S.P. when the master cylinder is in the rest position.
The piston according to the invention also allows the return of the corresponding chamber to an initial rest position to occur in such a way that, during this return, very few air bubbles are formed in the brake fluid contained in the corresponding chamber.
The piston according to the invention also allows the return of the corresponding chamber to an initial rest pressure to occur in such a way that, during this return, the corresponding seal is not extruded from its housing.
A subject of the invention is therefore a master cylinder for a motor vehicle and comprising                a body in which a bore is formed longitudinally with respect to an axis of the master cylinder,        a hydraulic fluid reservoir opening into the bore of the master cylinder,        a piston sliding in the bore and defining a chamber of the master cylinder, the piston comprising a port opening, at one end, to face a wall delimited by the bore, and, at the other end, opening into the chamber, the port receiving hydraulic fluid from the reservoir, and        a sealing means housed in a groove formed in the wall, the sealing means controlling the passage of hydraulic fluid through the port through collaboration between the sealing means and the piston, characterized in that        the piston comprises at least one indentation created upstream of the port in the piston in a direction of forward travel of the piston during a braking phase, the seal collaborating with the piston via the indentation.        